The present invention relates generally to photomultiplier tube assemblies and more particularly to photomultiplier tube assemblies used in high speed light actuated inspection apparatus.
Photomultiplier tubes of the type conventionally used in industry consist of a photocathode plate positioned at one end of a tubular housing and a chain of dynodes mounted within the housing behind the photocathode. A photon of light striking the forward surface of the photocathode causes it to emit a photo electron from its rear surface which is received by the forward most dynode. Each dynode is connected to an energy source and has the characteristic of emitting a predetermined number of electrons from its surface for each electron received on its surface. Thus each electron striking the surface of the first dynode causes the emission of multiple electrons which strike the surface of the second dynode which in turn emits multiple electrons to the next dynode in the chain. Thus a cascading effect is produced whereby the electron emission from the last dynode is equal to the product of the multiplier effects of each dynode. For example, in a photomultiplier tube having four dynodes each having a multiplier effect of 100 to 1 the total photomultiplier effect of the tube would be 100.times.100.times.100.times.100 or 10.sup.8 to 1.
The electron flow is collected by an anode and is thereafter amplified to produce a more easily measureable electrical signal. This electric signal may be further processed to provide an accurate indication of the amount of light entering the photomultiplier tube. Thus a photomultiplier tube may be used to accurately measure an extremely small quanta of light.
Photomultiplier tubes have been used extensively for the purpose of testing products for defects. One such application is described in U.S. Pat. No. 4,074,809 issued to McMillin et al., which is hereby incorporated by reference. In machines of this type can body members held in peripheral pockets of a rotating wheel are inspected at high speed--on the order of 1000 cans per minute. The can bodies have one open end and are inspected by placing the periphery of the open end in a light sealed enclosure with the interior of the can in light transmitting communication with the light receiving end of a photomultiplier tube. The exterior surface of the can is flooded with light and any light passing through a defect in the can body member such as a crack, pinhole, etc., is sensed by the photomultiplier tube. A can reject signal is generated at a predetermined level of light intensity associated with a defect of predetermined size. However, a problem has developed with the use of photomultiplier tubes in applications such as the McMillin et al. invention due to the response characteristics of conventional photomultiplier tubes to an over-lighted condition and also due to other "noise" produced by light leaks in the light sealed enclosure and machine heat, etc.
The dynodes of photomultiplier tubes used in applications such as McMillin et al., are extremely sensitive due to the application of the tube in detecting extremely small amounts of light passing through minute cracks, etc. Thus when a light seal about the can is improperly formed or when a can holding pocket is empty or contains a can having a gross defect, the photocathode is exposed to intense light causing the dynodes to become saturated with electrons. In such a condition, the dynodes continue to emit electrons for several seconds after exposure to the intense light even in the absence of other light, and thus produce a series of erroneous can reject signals. The erroneous reject signals cause subsequently tested cans to be rejected, whether or not defective, until the photomultiplier tube recovers from the intense light exposure. In testing machines such as McMillin et al., which are operating at speeds on the order of 1000 cans per minute, over 100 cans may be tested and improperly rejected during a typical recovery period of approximately ten seconds. Similarly, minute defects in the light sealing apparatus or machine heat transmitted to the photomultiplier tube may generate a low level "noise" signal which is, in effect, added to the amount of signal produced by light from a can defect. The addition of the noise to the signal causes a reject signal at lower light levels than that for which the system was designed, again resulting in unnecessary scrap.
In order to prevent unnecessary scrap in testing apparatus such as McMillin et al., and to improve the performance of photomultiplier tubes in other high speed applications it is desirable to provide a photomultiplier tube assembly which eliminates erroneous signals during the recovery time associated with over exposure to an intense light source and which automatically adjusts itself to account for small light leaks in the system sealing apparatus, machine heat, etc.